The present invention relates to an electronic circuit for linearization of an amplifier, which has a error signal amplifier, which amplifiers an error signal derived from the input and output signal of the amplifies, which is superimposed on the output signal for compensation of the intermodulation products contained in it.
This type of amplifier, which is designated in the literature as a feed-forward-amplifier, is, for example described in IEEE Transactions on Microwave Theory and Techniques, Vol. 2, No. 6, June 1994, pp. 1086 to 1087.
There are very strict requirements for the linearity of a transmitting amplifier for example for the QAM modulation type used for digital radio broadcasting. The higher the number of modulation stages (16, 64, 128, 256, 512 QAM) , the higher are the linearity requirements for the amplifier. The transmission power also increases for the higher modulation stages 256 and 512 QAM, so that a greater signal/noise ratio is obtained at the same radio field strength.
It is desirable to use as few amplifier stages as possible. Thus one tries to modulate each amplifier as much as possible, that is to bring the peak modulation in the vicinity of 1 dB-compression point. Thus the amplifier is not only operated in its linear, but also in its nonlinear, region. In order in spite of that to obtain a sufficiently large intermodulation spacing in the output signal of the amplifier, the above-mentioned linearization of the amplifier is performed. The error signal amplifier used in the feed-forward-amplifiers may of course only be operated in its linear control range so that it does not still produce additional intermodulation products. However a comparatively greater linear modulation range is then required from these error signal amplifiers, particularly when these error signals are large. An error signal amplifier with a comparatively large linear modulation range, i.e. also with a high 1 dB-compression point, is difficult to make so that it is comparatively expensive. This type of error amplifier produces a comparatively high power loss.